1. Field of the Invention
This invention relates to production of an electromagnetic wave by degenerate four-wave mixing using a multiple quantum well structure as a room temperature nonlinear optical element.
2. Description of the Prior Art
Heretofore at room temperature nonlinear optical materials have required more power than is available from a milliwatt range solid state laser in order to saturate their absorption and thereby cause their index of refraction to vary with incident light intensity.
Nonlinear effects of light intensity on the index of refraction may be represented by the equation EQU n=n.sub.L +n.sub.2 I (1)
where n is the index of refraction of the material, n.sub.L is the low intensity index of refraction, I is the intensity of the incident light beam in, for example, units of Watts/cm.sup.2, and n.sub.2 is the coefficient of nonlinearity of the material and may be expressed in the units cm.sup.2 /Watt.
The third order susceptibility X.sup.(3) gives a more general specification of the nonlinear properties of materials than does n.sub.2. n.sub.2 is related to the third order susceptibility.
Measurements of n.sub.2 at a wavelength of 1.6 microns for a number of materials were reported by Moran et. al. in the article "Interferometric Measurements of the Nonlinear Refractive Index Coefficient Relative to CS.sub.2 in Laser System Related Materials", in the IEEE Journal of Quantum Electronics, Vol. QE-11, June 1975, p. 259, and showed that n.sub.2 for the material CS.sub.2 is from 10 to 100 times larger than for a variety of materials used in laser construction. Moran et. al. give n.sub.2 =3.10.sup.-14 cm.sup.2 /Watt for CS.sub.2. A measurement of the third order nonlinear susceptibility X.sup.(3) of silicon at a wavelength of 1.06 microns was reported by Jain et. al. in the article "Degenerate Four-Wave Mixing Near the Bandgap of Semiconductors" in Applied Physics Letters, Vol. 35, September 1979, p. 454, as 8.10.sup.-8 esu, and this value is equivalent to a value of n.sub.2 =3.5.times.10.sup.-10 cm.sup.2 /Watt. The above values of n.sub.2 are too small to make the above materials useful in a nonlinear optical device in which a milliwatt diode laser is used as the light source.
Heretofore degenerate four wave mixing (DFWM) has been used for a number of optical processing applications. DFWM was used for correction of phase aberrations in laser amplifier systems, Bloom et. al. U.S. Pat. No. 4,220,928 issued September 1980, using CS.sub.2, or Nd:YAG as the nonlinear material. DFWM was used for a tunable optical filter, Bjorklund et. al. U.S. Pat. No. 4,198,162 issued April 1980, using atomic sodium vapor, ruby, or CS.sub.2 as the nonlinear medium. DFWM was used for a high-speed spectrally selective optical gate, Bjorklund et. al. U.S. Pat. No. 4,178,079 issued Dec. 1979, using CS.sub.2, retinol, retinol acetate, ruby, atomic metallic vapor, or Nd:YAG as the nonlinear material. DFWM was used for detection of birefringence in irregularly shaped objects, Bjorklund et. al. U.S. Pat. No. 4,352,566 issued October 1982, using atomic sodium vapor, Nd:YAG, ruby, BSO, or LiNbO.sub.3 as the nonlinear medium.
Four-wave mixing was used for making a tunable laser, Hodgson et. al. U.S. Pat. No. 3,816,754 issued June 1974, using a variety of metallic vapors, including potassium, rubidium, cesium, sodium, or lithium as the nonlinear material. Four-wave mixing was tuned by use of stark processes, Bjorklund et. al. U.S. Pat. No. 4,091,290 issued May 1978, using strontium vapor as the nonlinear material. Four-wave mixing was used for tuning of lasers in the infrared region, Begley et. al. U.S. Pat. No. 4,095,121 issued June 1978, using NH.sub.3, LH.sub.3 F, D.sub.2, HCL, HF, CO, or H.sub.2 as the nonlinear medium. Four-wave mixing using liquids because of their higher density was suggested by Kiddal et. al. U.S. Pat. No. 4,107,544 issued August 1978, using liquid N.sub.2 H.sub.2, O.sub.2, CO.sub.2, CO, NO, or CH.sub.4 as the nonlinear medium.
DFWM was used as a phase conjugate mirror by R. C. Lind et. al., as reported in the article "Demonstration of the Longitudinal Modes and Aberration Correction Properties of a Continuous-Wave Dye Laser with Phase Conjugate Mirror", Optics Letters, Vol. 6, pp. 554-556, Nov. 1981, in which sodium vapor was used as the nonlinear material.
Hegarty et. al. in the article "Resonant Degenerate Four-Wave Mixing in GaAs Multiquantum Well Structures," published in Applied Physics Letters, January 1982, at pp. 132-134, observed backward degenerate four-wave mixing with GaAs/AlGaAs multiple quantum well structures, but observed the effect only at temperatures below about 90.degree. K. The low temperature operation shown by Hegarty et. al. is impractical for commercial device utilization.
A common problem exhibited by all of the above nonlinear optical materials is that their third order nonlinear susceptibility is too small for a practical device using a diode laser light source.